Digital lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g., red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, which are hereby incorporated by reference.
There is a need for dimmable solid state lighting (SSL) units, such as retrofit SSL lamps, including LED lamps. The SSL lamps should be compatible with a wide range of existing dimmers. However, most existing dimmers have been designed for operation with incandescent light lamps. SSL lamp input characteristics typically differ from incandescent light bulbs, so interfacing circuitry is required for correct operation.
Many techniques have been proposed for configuring SSL lamps to enable “normal” dimmer operation. In other words, the techniques seek to emulate incandescent lamp behavior, e.g., by providing a low impedance current path during zero crossing. This allows auxiliary dimmer supply and dimmer timing circuit operation similar to the traditional load. However, control information regarding dimming (“dim information”) is received by the SSL lamp via a phase cut power signal, so control information and energy are incorporated in the same signal. Accordingly, the power signal must be separated into the control information part and the power part. Compromises in efficiency of reception and processing of the power signal (e.g., the above-mentioned low impedance path, often realized by lossy bleeders) are required to obtain stable and continuously available dim information.
Low cost dimmers are often based on simple resistor-capacitor (RC) timing circuits, where a variable resistor (potentiometer) charges a fixed capacitor. When the capacitor voltage reaches a threshold value, a power switch is activated or deactivated. The duration during which the power switch stays on is determined by the type of power switch, the load and/or other timing circuits. “Emulation” of an incandescent light bulb attempts to provide “normal” operation of the RC timing circuit. As mentioned above, the dimmer will provide a phase cut power signal to the lamp, containing both energy and dim information. Thus, the power signal delivered to the lamp may change from one (half-) cycle to the next, preventing continuous, stable operation of the timing circuit. Also, the desired level of light to be output by the SSL lamp as indicated by the conventional dimmer setting may not be properly translated to the SSL lamp, resulting in a level of output light that differs from the expected desired level of output light.
Thus, there is a need in the art for an SSL unit capable of providing continuous, stable operation during dimming operations, and of outputting a level of light consistent with the dimmer setting.